While details remain hazy, it is believed they took classified information away with them. Kockums' head of security was relieved of his duties on the same day.

On Thursday, lawyer Dennis Töllborg, a law professor, questioned the legality of the move.

"There is a particular crime called extraction (självtäkt) that has a slightly lower degree of penalty than theft," he told Sveriges Radio (SR). "You can't just go onto someone's land and take away your own possessions from someone else's control."

While the head of Thyssen Krupp denied that any information was taken from the premises, an FMV spokeswoman has confirmed that "a routine transportation of defence material" took place.

Earlier this week, Gunnar Hult, a professor at Sweden's National Defence College, told The Local that the move by FMV was legally murky. However, he thought the FMV would be able to escape without legal action.

"I suppose if they say it refers to national interest then they'll be able to get away with it," he added.